Major Histocompatibility Complex (MHC) molecules are proteins found on cell surfaces that play a key role in the immune system. They act as “identity tags” for the body’s cells, allowing the immune system to distinguish between healthy “self” cells and those that are foreign or abnormal, such as infected or cancerous cells. This distinction is fundamental for initiating an appropriate immune response. The specific location of these molecules on different cell types dictates how they interact with immune cells and contribute to immune surveillance.
MHC Class I Molecules: Found on Nearly All Cells
MHC Class I molecules are present on the surface of nearly all nucleated cells throughout the body. This widespread distribution includes cells like muscle, skin, liver, and most white blood cells. Red blood cells are a notable exception as they lack a nucleus and therefore do not express MHC Class I molecules.
Their ubiquitous presence allows the immune system to constantly monitor the internal state of almost every cell. These molecules primarily present small fragments, or “peptides,” of proteins that originate from within the cell. This includes peptides from normal cellular proteins, but also from abnormal proteins synthesized by infected or cancerous cells.
When a cell is infected by a virus or becomes cancerous, it produces foreign or altered proteins internally. Fragments of these proteins are loaded onto MHC Class I molecules and displayed on the cell surface. This presentation allows cytotoxic T lymphocytes to recognize and target these compromised cells for destruction.
MHC Class II Molecules: Specialized Immune Cell Presence
In contrast to MHC Class I, MHC Class II molecules have a more restricted distribution, found only on the surface of specific immune cells. These specialized cells are known as professional antigen-presenting cells (APCs). The primary types of professional APCs expressing MHC Class II include macrophages, dendritic cells, and B lymphocytes.
The restricted presence of MHC Class II molecules on these cells reflects their role in initiating specific immune responses. These APCs engulf and process foreign invaders, such as bacteria or toxins. After processing, fragments of these external pathogens are loaded onto MHC Class II molecules.
The MHC Class II-peptide complexes are then transported to the cell surface of the APC. Here, they are presented to helper T lymphocytes, which coordinate immune responses. This interaction activates helper T cells, which stimulate other immune cells, including B cells to produce antibodies and cytotoxic T cells.
The Importance of Cell Surface Location
The placement of both MHC Class I and Class II molecules directly on the cell surface is important for their immune function. This exposed position enables constant interaction with T lymphocytes. Without this surface display, T cells could not effectively detect internal or external threats.
MHC Class I molecules on virtually all nucleated cells provide a broad surveillance mechanism, indicating the health status of individual cells. If a cell displays foreign peptides via MHC Class I, it signals an internal problem, such as a viral infection, directly to cytotoxic T cells. This direct communication allows for the rapid elimination of compromised cells.
Similarly, the surface presentation of antigens by MHC Class II on professional APCs is important for initiating adaptive immunity against external pathogens. These APCs capture foreign material and present it to helper T cells to coordinate a targeted immune response. This surface communication is the primary way the immune system distinguishes between “self” and “non-self” and mounts specific defenses.